The rapid development of IP technology (Internet Protocol) during the past few years has expanded the feasible uses of different IP-based applications beyond the conventional Internet data transmission. IP-based telephone applications, in particular, have improved fast, and consequently a larger and larger portion of the call transmission path can in principle be implemented utilizing the IP technology both in traditional switched telephone networks (PSTN/ISDN, Public Switched Telephone Network/Integrated Services Digital Network) and in mobile communication networks (PLMN, Public Land Mobile Network).
In mobile communication networks, in particular, IP technology provides several advantages since in addition to traditional call services, which could be implemented by means of various IP speech applications, the mobile communication networks will offer more and more various data services, such as Internet browsing, e-mail services, games, etc., which are typically most advantageously implemented as packet-switched IP-based services. Thus the IP layers to be included in the protocols of mobile communication systems could serve both audio/video services and various data services.
In mobile communication networks it is particularly important to utilize the limited resources as efficiently as possible. However, this makes it more difficult to utilize IP protocols at the radio interface because, in IP-based protocols, the proportion of various headers in the data to be transferred may be very large, and consequently the proportion of payload is small. Furthermore, in poor conditions the bit error rate (BER) of the radio interface and the round-trip time (RTT) of the uplink and the downlink may increase a lot, which causes problems to most prior art header compression methods. For this reason, a need has arisen to devise a header compression method which is suitable for various IP protocols and for data transmission over the radio interface, in particular: efficient compression of headers which can be utilized in conditions where bit error ratios and delays increase a lot.
For this reason, the IETF (Internet Engineering Task Force) has recently standardized a header compression method known as the ROHC (Robust Header Compression). One of the underlying ideas of the ROHC development is that there is a lot of redundancy between the several IP headers used in data packet transfer, not only inside the data packets but also between them. In other words, most of the information in headers does not change at all during the transmission of data packets, in which case the information included in the headers can be easily reconstructed at the receiving end, even though it is not transmitted at all. Only a small number of headers include information which requires attention in compression. The ROHC further comprises several compression levels where the efficiency of compression increases always when compression shifts to an upper level. The ROHC always tries to use the most efficient compression available; however, before shifting to the next level, it is always ensured that the operation at the level in question is sufficiently reliable.
In the ROHC, several packet data flows transmitted on the same radio link are separated from one another by means of a context identifier CID. The context identifier is attached to a data packet to be transmitted and the receiver can determine the packet data flow and compression context the data packet belongs to from this context identifier. When applied to third generation mobile communication systems, particularly to the UMTS (Universal Mobile Telecommunication System), the context identifier CID can be attached to the data packet to be transmitted at least in two ways. The context identifier can be attached in an internal procedure of the ROHC, where, according to the ROHC specification, a compressed data packet comprises a specific CID field to which the context identifier CID is attached. The other alternative is to attach the context identifier to or associate it with a header of a data packet of a protocol layer of the mobile communication system, preferably to that of the convergence protocol layer, so that the compressed data packet does not include a context identifier CID in accordance with the ROHC specification, but the context identifier or another identifier associated with it is attached to the convergence data packet, particularly to the header of the data packet, into which the above-mentioned ROHC-compressed protocol packet has been fed.
UMTS specification (3GPP TS 25.323, version 4.0.0, paragraph 5.1.3) defines that when the ROHC compression is used, either one of the above-mentioned methods has to be selected, but the specification does not define the selection method. Thus there is a need to devise a method of configuring the UMTS system so that the above-mentioned selection can be carried out.